Note: Descriptions are shown in the official language in which they were submitted.
~N c995
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A method of manufacturing cladding tubes of a zirconium-based ~lloy for
The present invention relate6 to a method for manufacturing cladding tube~
of a ~irconium-based alloy for fuel rods for nuclear rsactors.
As cladding tubes for ~uel rods for nuclear reactors there are normally
used thin-walled tube~ of zirconium-baeed alloy~, known under the n~me
ZircaloyJ The~e alloy3 contain alloying materials such as tin, iron and
nickel. In Zircaloy the ~-pha~e i8 ~table below 79O C, the ~ -phaae i8
stable above 9~O C, whereas a two-phase re~ion, the C~ -phase region,
OCCUr9 between 79O C and 95O C. In the G~-pha~e the zirConiUm atoms are
arranged in a hexagonal close packed lattice and in the ~ -phase in a body
centered ~ubic lattioeO During so-called ~ -quenching of Zir~aloy*to bring abo~the de~ired properties 0ùch a3 improved oorro~lon properties of ~he ~aterial9
the material 9 heated to a temperature in the ~ -pha~e region and rapidly
: oooled to a temperature in the ~ pha~e region~
During conventional manufacture of cladding tubes o~ Zircaloyi a ~ quenching
of the ~aterial i~ performed after forging of i~got3 into rods, ~fter the.
~anufacture o~ extrusion billets of the rods; the billets ~re extxuded in the
phase region at a temperature of 680 C, wherea~ter the e~truded product iB
subjected to cold rollings in a ~u~ber o~ 8tep8 and, between two o~eGuti~e co
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rollings, an annealing, intermediate annealing, at 625-700 C to enable the
subsequent cold rolling. After the last cold rolling step, a final annealing
is performed to give the material the desired properties. ~he final annealing
may be ~erformed at temperatures of 400-700 C.
Tubes manufactured from Zircaloy under the conditions uæed so far have
generally proved to posses3 sufficient resistance to corrosion under the
operating conditions prevailing in a nuclear reactor. ~o~rever7 the develop-
ment proceeds towards an increasingly higher utilization of the fuel7 which
means longer operating times for the fuel assemblies. qhe claddin~ material
will therefore be subjected to the corrosive water for a longer perioB of
time than what ha~ been normal previously, which results in an increased
risk of corrosion damage. It has therefore been a desire to achieve better
corrosion properties in the alloys used without thi~ in~olving unfavourable
changes of the mechanical properties.
.~
It is previously known9 among other things from the U,S. patent specific~-
tion 4,238,251, that by ~-quenching of a finished tube of Zircaloy it is
possible to improve the resistance of the tube to so-called accelerated
nodular corro~ion in water and steam of high pressure As will be ¢lear from
U.S. patent specification ~,~65,635, tubes of Zixcaloy having good mechanical
properties may be achieved by ~-quenching of the extruded product before this
is subjected to the final cold rolling operation.
The exact reason for the improved resistance to accelerated nodular corrosion
achieved by ~ quenching has as yet not been completely established. It i8
considered, however, that the improvement is related to the ~ize and distri-
bution of the intermetallic compounds in the material~ ~he intermetallic
compounds, so-~alled second phases, consist of ¢hemical compounds containing,
beside~ zirconium, primarily the èlements iron~chromium and nickel and they
exist in the form of particles~ The dissolution and preGipitation process
a¢complished by the ~ quenching results in a reduction of the size of the
particles as well a~ in a redistributio~ from evenly distributed particle~
to particles constituting arrays at the grain boundaries of the ~-grain~
formed during the ~ phase transformation.
A ~quenching of the finished cladding tube results in a reduction of the
ductility of the tube, which involve~ ~ disadvantago with the method. A ~-
quenching of the extruded product prior to cold rolling into the final dimension
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gives leg9 deterioratio~ of the mechanioal propertie8 of the finished tube.
quenching, however, regardles~ of whether it takes place on a finished tube
or prior to the final cold rolling step, results in an impaired
yield because of the increa~ed ~mount of aCrap and further beGause Or
material losses since a ~quenchin6 leads to the formation of an oxide la~er
on the surface of the tube, which mu~t be removed.
i
Acoording to the present invention, it has proved to be po3sible to manufac-
ture cladding/for ~uel rods for nuclear reactors ha~ing at leaat as good
re~istance ~o nodular corrosion ao the be~t previously known cladding tube6
and at the sam~ time a better ductility than such cladding tubes. Compared
with previously known method3 of manufacturing cladding tubes while using ~ _
~uenching after the extrusion, u~in~ the present invention, which also in-
cludes ~ quenching, results in an improved yield because of reduced
s¢rap and further reduced material losseR since the oxides form2d
can be re~.oved on a smaller surface by performing the ~-quenching in an earlierstage of the manufacturin~ process.
~he invention relates to a method of manufacturin~ a cladding tube of a
zirconium-based alloy for fuel rod~ for nuclear reactors, the zirconium-
based allo~ being extruded and the extruded product bsing subjected to cold
rolling~ and at least one annealing, intermediate annealing, between two
consecutive cold rollings and a ~^luenching prior to the final oold rolling,
characterised in that the ~ quenching is performed prior to a cold rolling,
after which at least one intermediate annealing is pex~ormed at a temperature
of 500-675 C. The preferred temperature for the in~ermediate
annealing is 500-6~10 C, and the specially preferred temperature
is 550-600 C.
The extrusion ¢an be carried out at sn arbitra~y temperature in the c~-Pha80
region~ !
After the last cold rolling, the extruded product is subjected to a final
annealing at a temperature o~ ~400~675 GC ~ preferably at a tenlperature
of 400-610 C and especially preferably at a temperature of 550-
600C
~he ~-quenching of the extruded product i~ carried out by heatLng the product
to a temperature in the ~ phase region, suîtably to a temperature of 950-1250 Cand preferably to a tempexature of 1000-1150 C and thereafter rapidly oooling
it to a temperature in the o~_phase region. ~he cooling from the temperature
used in the ~-phase xegion to the temperature 790 C then guitably takes place
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a-t a rate of 20-400 C per second and the cooling from 790 C to 500 C or
a temperature therebslow suitably at a rate of more than 5 C per minute.
In the manufacture of cladding tubes according to the present invention, it
has pro~ed that the size of the second phase particles in the finished
cladding tube, as in the case where ~ uenching i~ u~ed, is considerably
smaller than with conventional manufacture of oladding tu~es ~lithout ~ -
quenching after the extrusion. Ho~lever, contrary to what i8 the caae after ~ -quenching in previously known methods, the second phase particles are homo-
geneously distributed in the material. It i9 pos~ible that it i9 the small
size of the second pha~e particles achieved according to the present invention,
together with the homogeneous distribution thereof, which gives the fa~ourable
combination of good resistance to nodular corrosion and good mechanical
properties.
The zirconium~ba~ed alloy preferably consists of a zirconium-tin alloy, for
example the alloy~ known under the trade name~ Zircaloy 2 and Zircaloy 4,
whose contents of alloying material lie within the l;m;ts 1.2-1.7 ~ for tin,
0.07-0 24 % for iron, 0005-0.15 % for chromium and 0-0,08 % for nickel, the
balance being zirconium and any existing Lmpurities of ordinary kind, the
percentages stated, as well as other percentages stated in the application,
referring to percentage by weight. Zircaloy 2 contains 1.2-1.7 ~0 tin, 0.07-
0.20 ~ iron, 0.05-0.15 % chromium,and 0.03-0.08 ~o nickel. Zircaloy 4 contains
1.2-1.7 % tin7 0.18-0.24 % iron, 0.07-0.13 % chromium and no nicXel.
The æirconium-ba~ed alloy i~ preferably subjected to a ~-quenching prior to
the extrusion, that is, it i~ heated to a temperature in the ~ phase region
and rapidly cooled to a temperature in the c~-phase region. ~oweverj it is
possible to use the zirconium based alloy without~`it b~ing subjected to ~ -
quenching. ~ -quenching prior to the extrusion i8 carried out by heating the
alloy to a temperature o~ suitably 950-1250 C and of preferably 1000-1150 C
ana by rapidly coaling it to a temperature in the o~-phase region. The
oooling from the u~ed temperature in the ~-phase region to the temperature
790 C then suitably takes place at a rate of 1-50 C per second and the cool-
ing from 790 C to 500 C or a temperature therebelow ~uitably at a rate of
more tha~ 5 C per mi~ute.
~he in~ention will be e~plained in greater detail by de~cribing an example.
An ing~t of Zircaloy 2 i~ forged into a rod with a dimension of 150-200 m~O
~he rod i8 subjected to ~ -quenching by heating it to a temperature o~ 1050 C
for 15 minutes a~d cooli~g it to room temperature at a rate of 5-10 & /~e¢ond.
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Ext~usion billets ~re made from the rod. ~hese billets are extruded at a
temperature of 700 - 740 C, that iæ, in the ~ ~hase region. The extruded
product iB thereafter subjected to three cold rolling operation~ whereby the
final outer diameter of the tube becomes 12.~ mm. ~etween the firs-t and the
second rolling the extruded product is subject0d to a ~ quenching by heating
it up to 1050 C for a few seconds with a high frequency coil arTanged around
it, whereafter it iæ cooled at a rate of 200 ~C/~eoond to room temperatuxe by
water spraying. Between the seco~d and last rolling, the ext~uded product i3
annealed at a temperature of 575 C. After the last cold rolling, the
tube is finally annealed at a temperature of 565 C, Both intermediate
annealings and the final annealin~ may be carried out in an evacuated
furnace. ~ In the ~inished tube, the second phase particles have a
size which i8 substantially in the interval 0.05-0.4/um and a mean particle
size of around 0015 ~ . In a cladding tube which has been manufactured in a
conventional manner and which has not been subjected to ~-quenching in finished
state or earlier in extruded state, the second phase particles have a size whichis substantially in the interval 0.1-0.6/um and a mean particle size of about
00 3/um.
During corrosion tests which have proved to simulate well the conditions
in reactor operation9 cladding tube~ manufactured according to the present
inventian exhibit ueight gain which i8 only a ~raction of that
which is obtained in conventional manufacture without the use of ~ quenching
after the extrusion and approximately as great as that obtained during
manufacture while using ~quenching after the extrusion, 50-100 mg/dm2
according to the invention and 350 4000 mg/dm2 during co~ventional manufac-
ture without the use of ~-quenching. ~he ductility of a cladding tube
manufactured according to the invention is better than with tubes which have
been sub~ected to ~uenching in finished atate and with tubes which have
been subjected to ~quenching immediatel~ prior to the last cold rollin~.
Ihe above-mentioned corrosion tests are performed in an autoclave with water
vapour at a pressure of 9.8 MPa and a temperature of 500 ~. The weight
gain i~ a mea~ure of the corrosion to which the tube has been ~ubjeQted.
